1. Field of the Invention
The invention relates to a damper device having a mass portion and an elastic portion, in particular, relates to a damper device incorporating a plurality of torsional dampers having different characteristics and together supported on a rotational element.
2. Description of the Related Art
One of damper devices that damp vibrations through dynamic vibration absorption at resonance frequencies is a torsional damper that damps torsional vibrations using mass portions and elastic portions, and this torsional damper is, for example, provided at the rotation input portion of a drive-force transfer unit, such as a differential unit for vehicles.
As one of damper devices of the kind described above, Japanese Utility Model Application Publication No. 61-109941 (JP-U-61-109941) describes a disk-shaped torsional damper that is constituted of two annular mass portions having different radiuses and arranged concentrically and two annular elastic portions having different radiuses and arranged between the two mass portions and between one of the two mass portions and a shaft portion, respectively. This torsional damper provides two characteristic vibrations through coupled vibrations of the two annular inertia portions each constituted of one mass portion and one elastic portion.
Further, Japanese Utility Model Application Publication No. 2-72835 (JP-U-2-72835) describes a damper device that has a double-layered damper structure constituted of a first vibration ring that is supported on the outer peripheral side of a hub via a first elastomer ring, a pulley that is provided on the outer side of the first vibration ring so as to provide an inertia mass required for vibration damping, and a second vibration ring that is disposed, via a second elastomer ring, in the inner space of the damper device.
Further, Japanese Utility Model Application Publication No. 6-69486 (JP-U-6-69486) describes a damper device in which first and second mass portions each tapered at the inner peripheral side thereof are attached, via tapered elastic portions, on tapered slide hubs that are opposed to each other. According to this damper device, the natural frequency of the damper device is adjusted as needed by changing the compression amounts of the elastic portions by moving the slide hubs relative to each other in the axial direction.
Further, Japanese Utility Model Application Publication No. 62-200033 (JP-U-62-200033) describes a differential unit for vehicles. According to this publication, based on the fact that the torsional vibration frequency that causes meshing noises at the final reduction gear is different between when the vehicle is being accelerated and when the vehicle is being decelerated, two torsional dampers having different resonance frequencies are concentrically arranged on the outer peripheral face of a flange provided at the input portion of the differential unit.
According to the technologies described in the above publications, however, it is still difficult to provide a compact damper device that provides desired vibration-damping effects for multiple target resonances having resonance frequencies largely different from each other.
For example, among various noises and vibrations that occur in a rear-drive vehicle, thrumming noises that are caused by resonances at the rotation input portion of a rear-differential unit and can be heard in the vehicle compartment are very difficult to suppress. The forces that cause such resonances at the rotation input portion of the rear differential unit (i.e., the mesh-point dynamic rigidity that represents the ratio of the amplitude of the cyclical response displacement of the rotation input portion of the rear differential unit to the amplitude of the vibration at the mesh point at the final reduction gear) can be suppressed by providing a related-art torsional damper at a flange of the rotation input portion of the rear differential unit. However, in order to suppress the aforementioned thrumming noises reliably, it is also necessary to suppress bending resonances at the rotation input portion of the rear differential unit.
However, usually, there is a large difference (e.g., a difference of several hundreds hertz) between the resonance frequency of a damper device that is required to provide a sufficient vibration-damping effect for torsional resonances and the resonance frequency of the damper device that is required to provide a sufficient vibration-damping effect for longitudinal prying resonances. According to related-art technologies, therefore, it is still difficult to provide a damper device that provides desired vibration-damping effects for two target resonances largely different in frequency but is compact enough to be fit in a limited space at the input portion of a differential unit.